The journey to find the truth behind the kidnapping of the five British men kidnapped in Iraq in May 2007 has been a long one. We at Guardian Films set out to find out why they had become the forgotten hostages. Was it because they were security men and a computer programmer working for private military companies in Iraq? Was it just because they lacked the glamour and profile of the BBC's Alan Johnson, Terry Waite or John McCarthy? Or was there another reason?

For the past seven years, Guardian Films has specialised in Iraq and over those years we have built up extensive contacts within the Iraqi intelligence service.

We began with that network on the search for the man who held the file on the taking of the five British hostages. He had become a political refugee in a European city. We set about persuading him to tell us all he knew of the kidnapping. It was during a four-hour interview that he revealed his men had witnessed the hostages being handed over at a remote brick factory near the Iranian border at Mehran.

But his vivid account was only one man's claims. We knew the only way to verify his testimony that the men had been taken to Iran was to find a source within Iran's al-Quds Force who would be prepared to testify for our film.

The search took us to another European city where we found a man with close links to Iranian intelligence. We believe that he would not have spoken were it not for his personal disillusionment with the regime following its bloody crackdown after last June's election .

He said that he had taken part in kidnappings and knew those involved in the abduction of the five Britons. He also agreed to reveal to us how al-Quds Force operates.

The interview took place in a hotel room and lasted two hours . He revealed to us that the kidnapping of the British men had taken place following the failure of a similarly audacious attempt, by al-Quds Force, to kidnap five American soldiers from a fortified US base in Karbala, southern Iraq. That attempt took place three months before they came for the Britons in Baghdad.

The attack on the American base at Karbala in January 2007 ended with al-Quds Force killing the five American soldiers they intended to take hostage.

The murders of the five soldiers were brutal. The grotesque nature of their deaths was one reason the Americans were loth to release the man who masterminded the attack on Karbala and the leader of all the Iranian "special groups" operating in Iraq: Qais al-Khazali. Unfortunately for the five British hostages their fate rested on his release.

During our investigation we also obtained copies of two internal documents from the kidnapped men's employers, GardaWorld and Bearing Point, both private military security companies operating in Iraq. Both these internal documents concluded that the men could have been taken to Iran.

As part of the 18-month investigation we were also scouring Arabic websites, including that of the Iranian proxy militia group the Righteous League, for any traffic or information on the fate of the British hostages.

In March we came across a report on a Saudi-based and funded website called Elaph that gave details about the kidnap negotiations.

The author clearly knew more than anyone else writing about the kidnapped men. He also appeared to have inside knowledge of the attempts being made to persuade the Americans to release their prize captive, Khazali

The author of the report specialised in Iraqi militia groups and said he could reach people who had taken part in the kidnap. It was not until August that he finally made contact with one of the kidnap team.

They denied Iran's involvement in the operations and insisted the Righteous League had worked alone, but his account was contradicted by several other sources.

The strongest confirmation that we were on the right track regarding al-Quds Force's involvement in the kidnapping came one Sunday morning in June in an elegant suburban house in London during a meeting with an Iraqi government minister who had taken part in the hostage negotiations and was himself closely connected to Iran. "The kidnapping of the British was the work of the Iranian Revolutionary Guard.

"You don't think that Iraqis from Sadr City [a Shia district of Baghdad] are capable of carrying out an operation like that," he told us.

During the last six months of the investigation the information from our sources in Iraqi intelligence pointed to the fact that a breakthrough was expected. They were convinced that the Americans, as they were preparing their exit strategy from Iraq, would finally release Khazali.

But the kidnappers had one final card to play before he would be released. They still had Moore.

In November the chief Iraqi negotiator, Sami al-Askari, told us the process was moving in the right direction and asked us to pass a message on to Moore's mother, Avril Sweeney, "that her son is alive and she will see him soon". Moore, unlike three of his four colleagues, survived. The fate of the fourth hostage, Alan McMenemy, is unconfirmed.

The news that Askari is denying any Iranian involvement in their kidnapping comes as no surprise to those of us that have been investigating the case. During the 18 months we spent on the investigation, the fact that the kidnapping was masterminded by al-Quds Force, has been something that anyone close to the current government in Iran has consistently denied.

Askari spent many years in exile in Iran as a political opponent of Saddam Hussein. He was selected as a negotiator precisely because of his closeness to the Iranian government.

For him to deny al-Quds Force's involvement in the murder of three British citizens, Alec Maclachlan, Jason Swindlehurst, and Jason Cresswell, and the cruel detention of Peter Moore and Alan McMenemy is to be expected.

Following a year-long investigation, GuardianFilms has exclusively uncovered the story of what happened to the five Britons kidnapped in Iraq on 29 May 2007, as the only survivor, Peter Moore, is released